1. Field of the Invention
The present invention relates to a process for priming a polymeric film with an effective amount of a primer coating composition on at least one side thereof to improve its bonding to silicone release compositions, and a process for bonding a silicone release coating to polymeric film. The primer coating consists of a copolyester composition uniformly blended with a glycidoxy silane. It has been found that this primer coating composition is excellent for the field of silicone release films and yields superior adhesion performance over known silicone release films.
2. Prior Art
Release films are well known and are generally employed to serve as a temporary support for an adhesive or tacky surface such as labels, pressure sensitive tape, decorative laminates, transfer tape, etc. Silicone compositions have long been used as the release coating for rendering the support surface (usually a paper support) relatively non-adherent to adhesive materials. Using various polymeric films as temporary supports is well known but most polymeric materials are virtually inert, and thus it is difficult to obtain adhesion between the silicone release coating and the polymeric film. Commercially available silicone release films initially have acceptable adhesion performance (adhesion between the support and the silicone release coating). However, with time the adhesion of the silicone release coating lessens which causes the release coating to rub off. This may cause silicone release material to be removed from the support when the label is peeled away, and thus the label may fail to properly stick. Also, the poor adhesion of the silicone release coating to the support may cause the silicone release coating to detach from the support, especially upon further processing of the coated film. This may cause the label to stick to the support and not properly detach for placement.
Prior commercial practice for manufacturing silicone release film does not involve employing primer coatings on the support. Typically, manufacturers have relied on silicone producers to formulate release compositions capable of adhering to the manufacturer's support. Consequently, there are many different types of silicone release compositions for many different supports.
Primer coating compositions for polymeric film supports are well known for product applications other than silicone release films. Primer coatings based upon polyolefins, polyesters, vinyls, alcohols, acids and acrylates are well known for film applications in the packaging industry, magnetic tape industry, and reprographic film industry. Exemplary of a known primer coating for the packaging industry is the following patent.
U.S. Pat. No. 4,493,872 to Funderburk et al (assigned to the present assignee) discloses a copolyester primer coating which may be applied to a polymeric film such as polyamides, polyolefin, polysulfones, polyacetal, polycarbonate and polyester (such as polyethylene terephthalate). This copolyester coating composition, when applied to a polymeric support, provides excellent adhesion between the polymeric support and a subsequently applied metallic coating.
The Funderburk et al copolyester primer coating composition consists of:
a) about 65 to 95 mol percent of isophthalic acid; PA1 b) about 0 to 30 mol percent of at least one aliphatic dicarboxylic acid; PA1 c) from about 5 to about 15 mol percent of at least one sulfomonomer; and PA1 d) an alkylene glycol having from about 2 to 11 carbon atoms. PA1 from about 40 to 98 mol percent isophthalic acid; PA1 from about 0 to about 50 mol percent of at least one aliphatic dicarboxylic acid; PA1 from about 2 to about 20 mol percent of at least one sulfomonomer containing a sulfonate group attached to a dicarboxylic nucleus; and PA1 about 100 mol percent of glycol having from about 2 to 11 carbon atoms; and PA1 drying said primer coating.
One approach for applying a primer coating to a polymeric support for silicone release applications is Japanese patent 64-5838 laid open on Jan. 10, 1989. This patent discloses a release film comprising a polyester film, a primer layer applied to the polyester film which consists of a crosslinkable silane coupling agent, and the silicone release layer applied to the silane coupling agent. The silane coupling agent may consist of a vinyl, epoxy (glycidoxy), amino, or mercapto silane. Once the silane primer coating has been applied and crosslinked with the polyester support film, it is then ready for uniform application of the silicone release coating.
Neither of the above references provide sufficient adhesion between the silicone release coating and the primed film such that the silicone release coating will not rub off. It is an aim of the present invention to provide a process of priming a polymeric support film with an effective amount of a primer coating to improve its bonding to a silicone release coating.
It is known that certain silicone release coatings will initially have excellent bonding to the polymeric support film, but this characteristic deteriorates rapidly (usually within one to seven days) such that the release film is unusable for long term applications.
An object of the present invention is to provide a process for priming a release film with a primer coating which maintains its original bonding characteristics to the silicone release coating for a sufficient period of time.
There are many different formulations of known silicone release coatings, such as crosslinked formulations, ultra violet curable or heat curable formulations, solvent, and solventless formulations, and combinations of these, such as solventless--u.v. curable formulations.
It is an aspect of the present invention to provide a process for priming a polymeric support with a primer coating which is compatible with and bonds well to a broad range of silicone release coatings as well as different polymeric supports.